Well, if I had a sheet metal brake, I would have definately used sheetmetal, or aluminum diamond plate for the main box.
Again, this was more of an experiment, and I have more wood tools than metal. The base of the box is masonite, and by itself really doesn't weigh much at all. I am planning on doing the top part with the lid in aluminum diamond plate though. That stuff's price really goes up depending on thickness. I will probably just cover the entire top with some thin stuff and leave it at that.

My rollbar,...
I cut the 2 side support bars at the top of the main hoop, then just flipped it around. I am going to bend the 2 side bars to go up at its existing angle, then bend those so they run parallel to the top of the bed, then re-weld them to the main hoop.